The suprachiasmatic nucleus (SCN) is the master circadian pacemaker in the mammalian brain. It generates robust circadian behaviors, entrained to environmental cues such as light input. To date, little is known about how firing from SCN neurons integrates input and generates coherent output rhythms in circadian behaviors such as wake/sleep, hormone levels, metabolism, and mood. The SCN is composed of various populations of genetically distinct neurons including vasoactive intestinal polypeptide (VIP) - expressing neurons. This proposal aims to understand how the neural code from VIP-expressing neurons of the SCN alters firing rate, circadian gene expression and locomotor behavior. I will identify the neural code of VIP SCN neurons and use optogenetic and pharmacological manipulations to test the role of distinct neurotransmitter signaling pathways. Specifically, I will identify characteristic VIP firing patterns during early subjective day (Aim I. Using these patterns, I can stimulate VIP neurons and test changes in SCN firing activity (Aim II), circadian gene expression (Aim III) and locomotor behavior (Aim IV). In addition, the use of pharmacological antagonists will allow me to parse out the role of VIP and GABA in these dual-transmitter expressing neurons. My preliminary data establishes the feasibility of these approaches; I demonstrated that ChR2 can be selectively expressed within VIP SCN neurons, stimulation can generate action potentials at physiologically relevant frequencies, and 15Hz 30min stimulation may cause decreases in gene expression synchrony and fragmentation of locomotor behavior. Overall, this proposal expands on my current research by answering key questions about how the endogenous neural code in VIP neurons sustains circadian synchrony and output. This is an important proposal because the results can help understand circadian disruption associated with neurological disorders, aging, and jet lag.